1. Technical Field
This invention generally relates to electrophotography, and more particularly, this invention relates to management and control of both a photoconducting belt and the liquid toner applied to the belt.
2. Background Art
Two problems, which are encountered when using a belt as a photoconductor in a liquid electrophotographic print engine, are solved by this invention. The first problem is proper belt tracking. If a belt is placed over two parallel rollers and rotated by means of rotating one of the rollers, the belt will tend to move towards one end of the rollers over time. This is a fact well recognized in the industry. This movement must be prevented or limited to prevent the belt from moving an excessive distance from its intended position. Many methods are used in the industry to prevent belt movement. Some of these are crowning of the rollers, belt edge contact, dynamic steering of one of the rollers, sprocket drive mechanisms, and tracking rails fixed to the belt that run in grooves in the rollers. This invention makes use of the last mentioned method.
The second problem that this invention addresses is unique to liquid electrophotography. After the liquid image is placed on the belt, a squeegee roller is used to press off excessive toner and to press the image tighter onto the belt surface. When using a roller that is shorter than the belt width, the fluid being pressed ahead of the squeegee roller can escape from the ends of the rollers. This fluid can then be pulled from the ends of the roller into the area behind the squeegee by capillary action. This liquid ruins the image that has been created and the lost fluid reduces the life of the consumable toner cartridge. There is a need to control these edge effects.
FIGS. 1 and 2 of the drawings illustrate a typical prior art belt-type photoconductor 2 employed in electrophotographic devices such as a laser printer 1. FIG. 1 is a simplified block diagram schematic of a liquid toner laser printer 1 having a belt-type photoconductor 2 on which four separate developers, magenta developer 7, yellow developer 8, cyan developer 9 and black developer 10, act to deposit toner. The first step in the color image forming process, at least in this simplified explanation, is to deposit a charge pattern on the photoconductor. This is accomplished by laser 11 which is modulated by a data stream representing color separated components of a color image. Starting with the yellow bit map first, laser 11 deposits a charge pattern on belt 2 which represents the full page yellow component image. Yellow toner is then applied to belt 2 by yellow developer unit 8. A squeegee roller 12, within each developer unit, is applied to belt 2 to remove any excess liquid toner which may accumulate on belt 2 and affix the image to belt 2. This process is repeated for each of the cyan, magenta and black component images, after which the entire image is transferred to the print media via transfer roller 13.
FIG. 2 is a detail partial cross-section view of the photoconducting belt taken across the length of the belt. The photoconducting belt 2 of the prior art generally has an organic photoconducting material impregnated in, or otherwise bonded to, a plastic film substrate such as Mylar.TM.. A pair of tracking ridges 3 are formed from an elastomeric material along the inside marginal edges of belt 2 to interact with tracking grooves 6 on either the drive roller 4 or idler roller 5 of the apparatus, with clearance grooves on the other roller. Accurate belt registration and tracking is critical in electrophotographic devices employing belt photoconductors and is especially critical if a belt-type photoconductor is to be used in color printing where multiple passes of the belt are necessary in order to deposit the four component colors onto the belt to form the image.
An additional concern in liquid electrophotography is preventing excess liquid toner which accumulates in the nip area of squeegee rollers 12 from migrating around the ends of the squeegee rollers and contaminating the exposed image area of belt 2. It is therefore a primary object of this invention to provide an accurate belt tracking mechanism which doubles as a mechanism to inhibit excess liquid toner from contaminating the deposited image on the belt.